Just for reference: I am running 9.97.1 RPi4.arm. Adjust refresh rate die not work for me either. Like described above i can see a part of the subs when setting "subtitle position on screen" to "above video". The subs then appear mid-right screen but only first view words of the line are visible...
EDIT: Disabling PRIME rendering does fix the issue -independent of the setting of "adjust refresh rate". Thanks for pointing to the thread!